Method and apparatus for imaging three-dimensional structure

ABSTRACT

An apparatus for determining surface topology of a portion ( 26 ) of a three-dimensional structure is provided, that includes a probing member, an illumination unit, a light focusing optics, a translation mechanism, a detector and a processor.

CROSS-REFERENCE

This application is a Continuation of U.S. application Ser. No.11/652,055, filed on Jan. 11, 2007, which is a Continuation of U.S.application Ser. No. 11/377,403, filed on Mar. 17, 2006, which is aContinuation of U.S. application Ser. No. 11/175,186, filed on Jul. 7,2005, which is a Continuation of U.S. application Ser. No. 10/692,678,filed on Oct. 27, 2003, which is a Divisional Application of U.S.application Ser. No. 09/775,298, filed on Feb. 1, 2001, which is aContinuation of PCT/IL99/00431, filed on Aug. 5, 1999, which claimspriority from Israeli Patent Application No. 125659, filed on Aug. 5,1998.

FIELD OF THE INVENTION

This invention in the field of imaging techniques and relates to amethod and an apparatus for non-contact imaging of three-dimensionalstructures, particularly useful for direct surveying of teeth.

BACKGROUND OF THE INVENTION

A great variety of methods and systems have been developed for directoptical measurement of teeth and the subsequent automatic manufacture ofdentures. The term “direct optical measurement” signifies surveying ofteeth in the oral cavity of a patient. This facilitates the obtainmentof digital constructional data necessary for the computer-assisteddesign (CAD) or computer-assisted manufacture (CAM) of toothreplacements without having to make any cast impressions of the teeth.Such systems typically includes an optical probe coupled to an opticalpick-up or receiver such as charge coupled device (CCD) and a processorimplementing a suitable image processing technique to design andfabricate virtually the desired product.

One conventional technique of the kind specified is based on alaser-triangulation method for measurement of the distance between thesurface of the tooth and the optical distance probe, which is insertedinto the oral cavity of the patient. The main drawback of this techniqueconsists of the following. It is assumed that the surface of the toothreflects optimally, e.g. Lambert's reflection. Unfortunately, this isnot the case in practice and often the data that is obtained is notaccurate.

Other techniques, which are embodied in CEREC-1 and CEREC-2 systemscommercially available from Siemens GmbH or Sirona Dental Systems,utilize the light-section method and phase-shift method, respectively.Both systems employ a specially designed hand-held probe to measure thethree-dimensional coordinates of a prepared tooth. However, the methodsrequire a specific coating (i.e. measurement powder and white-pigmentssuspension, respectively) to be deposited to the tooth. The thickness ofthe coating layer should meet specific, difficult to controlrequirements, which leads to inaccuracies in the measurement data.

By yet another technique, mapping of teeth surface is based on physicalscanning of the surface by a probe and by determining the probe'sposition, e.g. by optical or other remote sensing means, the surface maybe imaged.

U.S. Pat. No. 5,372,502 discloses an optical probe for three-dimensionalsurveying. The operation of the probe is based on the following. Variouspatterns are projected onto the tooth or teeth to be measured andcorresponding plurality of distorted patterns are captured by the probe.Each interaction provides refinement of the topography.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for imagingthree-dimensional structures. A preferred, non-limiting embodiment, isconcerned with the imaging of a three-dimensional topology of a teethsegment, particularly such where one or more teeth are missing. This mayallow the generation of data for subsequent use in design andmanufacture of, for example, prosthesis of one or more teeth forincorporation into said teeth segment. Particular examples are themanufacture of crowns or bridges.

The present invention provides, by a first of its aspects, a method fordetermining surface topology of a portion of a three-dimensionalstructure, comprising:

-   (a) providing an array of incident light beams propagating in an    optical path leading through a focusing optics and a probing face;    the focusing optics defining one or more focal planes forward said    probing face in a position changeable by said optics, each light    beam having its focus on one of said one or more focal plane; the    beams generating a plurality of illuminated spots on the structure;-   (b) detecting intensity of returned light beams propagating from    each of these spots along an optical path opposite to that of the    incident light;-   (c) repeating steps (a) and (b) a plurality of times, each time    changing position of the focal plane relative to the structure; and-   (d) for each of the illuminated spots, determining a spot-specific    position, being the position of the respective focal plane, yielding    a maximum measured intensity of a respective returned light beam;    and-   (e) based on the determined spot-specific positions, generating data    representative of the topology of said portion.

By a further of its aspects, the present invention provides an apparatusfor determining surface topology of a portion of a three-dimensionalstructure, comprising:

-   -   a probing member with a sensing face;    -   an illumination unit for providing an array of incident light        beams transmitted towards the structure along an optical path        through said probing unit to generate illuminated spots on said        portion;    -   a light focusing optics defining one or more focal planes        forward said probing face at a position changeable by said        optics, each light beam having its focus on one of said one or        more focal plane;    -   a translation mechanism coupled to said focusing optics for        displacing said focal plane relative to the structure along an        axis defined by the propagation of the incident light beams;    -   a detector having an array of sensing elements for measuring        intensity of each of a plurality of light beams returning from        said spots propagating through an optical path opposite to that        of the incident light beams;    -   a processor coupled to said detector for determining for each        light beam a spot-specific position, being the position of the        respective focal plane of said one or more focal planes yielding        maximum measured intensity of the returned light beam, and based        on the determined spot-specific positions, generating data        representative of the topology of said portion.

The probing member, the illumination unit and the focusing optics andthe translation mechanism are preferably included together in onedevice, typically a hand-held device. The device preferably includesalso the detector.

The determination of the spot-specific positions in fact amounts todetermination of the in-focus distance. The determination of thespot-specific position may be by measuring the intensity per se, ortypically is performed by measuring the displacement (S) derivative ofthe intensity (I) curve (dI/dS) and determining the relative position inwhich this derivative function indicates a maximum maximum intensity.The term “spot-specific position (SSP)” will be used to denote therelative in-focus position regardless of the manner in which it isdetermined. It should be understood that the SSP is always a relativeposition as the absolute position depends on the position of the sensingface. However the generation of the surface topology does not requireknowledge of the absolute position, as all dimensions in the cubic fieldof view are absolute.

The SSP for each illuminated spot will be different for different spots.The position of each spot in an X-Y frame of reference is known and byknowing the relative positions of the focal plane needed in order toobtain maximum intensity (namely by determining the SSP), the Z or depthcoordinate can be associated with each spot and thus by knowing theX-Y-Z coordinates of each spot the surface topology can be generated.

In accordance with one embodiment, in order to determine the Zcoordinate (namely the SSP) of each illuminated spot the position of thefocal plane is scanned over the entire range of depth or Z componentpossible for the measured surface portion. In accordance with anotherembodiment the beams have components which each has a different focalplane. Thus, in accordance with this latter embodiment by independentdetermination of SSP for the different light components, e.g. 2 or 3with respective corresponding 2 or 3 focal planes, the position of thefocal planes may be changed by the focusing optics to scan only part ofthe possible depth range, with all focal planes together covering theexpected depth range. In accordance with yet another embodiment, thedetermination of the SSP involves a focal plane scan of only part of thepotential depth range and for illuminated spots where a maximumilluminated intensity was not reached, the SSP is determined byextrapolation from the measured values or other mathematical signalprocessing methods.

The method and apparatus of the invention are suitable for determining asurface topology of a wide variety of three-dimensional structures. Apreferred implementation of method and apparatus of the invention are indetermining surface topology of a teeth section.

In accordance with one embodiment of the invention, the method andapparatus are used to construct an object to be fitted within saidstructure. In accordance with the above preferred embodiment, such anobject is at least one tooth or a portion of a tooth missing in theteeth section. Specific examples include a crown to be fitted on a toothstump or a bridge to be fitted within teeth.

By one embodiment of the invention, the plurality of incident lightbeams are produced by splitting a parent beam. Alternatively, eachincident light beam or a group of incident light beams may be emitted bya different light emitter. In accordance with a preferred embodiment,light emitted from a light emitter passes through a diffraction orrefraction optics to obtain the array of light beams.

In accordance with one embodiment, the parent light beam is lightemitted from a single light emitter. In accordance with anotherembodiment, the parent light beam is composed of different lightcomponents, generated by different light emitters, the different lightcomponents differing from one another by at least one detectableparameter. Such a detectable parameter may, for example be wavelength,phase, different duration or pulse pattern, etc. Typically, each of saidlight components has its focus in a plane differently distanced from thestructure than other light components. In such a case, when the focalplane of the optics is changed, simultaneously the different ranges ofdepth (or Z component) will be scanned. Thus, in such a case, for eachilluminated spot there will be at least one light component which willyield a maximum intensity, and the focal distance associated with thislight component will then define the Z component of the specific spot.

In accordance with an embodiment of the invention the incident lightbeams are polarized. In accordance with this embodiment, typically theapparatus comprises a polarization filter for filtering out, from thereturned light beams, light components having the polarization of theincident light, whereby light which is detected is that which has anopposite polarization to that of the incident light.

The data representative of said topology may be used for virtualreconstruction of said surface topology, namely for reconstructionwithin the computer environment. The reconstructed topology may berepresented on a screen, may be printed, etc., as generally known perse. Furthermore, the data representative of said topology may also beused for visual or physical construction of an object to be fittedwithin said structure. In the case of the preferred embodiment notedabove, where said structure is a teeth section with at least one missingtooth or tooth portion, said object is a prosthesis of one or moretooth, e.g. a crown or a bridge.

By determining surface topologies of adjacent portions, at times fromtwo or more different angular locations relative to the structure, andthen combining such surface topologies, e.g. in a manner known per se, acomplete three-dimensional representation of the entire structure may beobtained. Data representative of such a representation may, for example,be used for virtual or physical reconstruction of the structure, may betransmitted to another apparatus or system for such reconstruction, e.g.to a CAD/CAM apparatus. Typically, but not exclusively, the apparatus ofthe invention comprises a communication port for connection to acommunication network which may be a computer network, a telephonenetwork, a wireless communication network, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1A and 1B are a schematic illustration by way of a block diagramof an apparatus in accordance with an embodiment of the invention (FIG.1B is a continuation of FIG. 1A);

FIG. 2A is a top view of a probing member in accordance with anembodiment of the invention;

FIG. 2B is a longitudinal cross-section through line II-II in FIG. 2A,depicting also some exemplary rays passing therethrough;

FIG. 3 is a schematic illustration of another embodiment of a probingmember; and

FIG. 4 is a schematic illustration of an embodiment where the parentlight beam, and thus each of the incident light beams, is composed ofseveral light components, each originating from a different lightemitter.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference is first being made to FIGS. 1A and 1B illustrating, by way ofa block diagram an apparatus generally designated 20, consisting of anoptical device 22 coupled to a processor 24. The embodiment illustratedin FIG. 1 is particularly useful for determining the three-dimensionalstructure of a teeth segment 26, particularly a teeth segment where atleast one tooth or portion of tooth is missing for the purpose ofgenerating data of such a segment for subsequent use in design ormanufacture of a prosthesis of the missing at least one tooth orportion, e.g. a crown or a bridge. It should however be noted, that theinvention is not limited to this embodiment, and applies, mutatismutandis, also to a variety of other applications of imaging ofthree-dimensional structure of objects, e.g. for the recordal orarcheological objects, for imaging of a three-dimensional structure ofany of a variety of biological tissues, etc.

Optical device 22 comprises, in this specific embodiment, asemiconductor laser unit 28 emitting a laser light, as represented byarrow 30. The light passes through a polarizer 32 which gives rise to acertain polarization of the light passing through polarizer 32. Thelight then enters into an optic expander 34 which improves the numericalaperture of the light beam 30. The light beam 30 then passes through amodule 38, which may, for example, be a grating or a micro lens arraywhich splits the parent beam 30 into a plurality of incident light beams36, represented here, for ease of illustration, by a single line. Theoperation principles of module 38 are known per se and the art and theseprinciples will thus not be elaborated herein.

The light unit 22 further comprises a partially transparent mirror 40having a small central aperture. It allows transfer of light from thelaser source through the downstream optics, but reflects lighttravelling in the opposite direction. It should be noted that inprinciple, rather than a partially transparent mirror other opticalcomponents with a similar function may also be used, e.g. a beamsplitter. The aperture in the mirror 40 improves the measurementaccuracy of the apparatus. As a result of this mirror structure thelight beams will yield a light annulus on the illuminated area of theimaged object as long as the area is not in focus; and the annulus willturn into a completely illuminated spot once in focus. This will ensurethat a difference between the measured intensity when out-of- andin-focus will be larger. Another advantage of a mirror of this kind, asopposed to a beam splitter, is that in the case of the mirror internalreflections which occur in a beam splitter are avoided, and hence thesignal-to-noise ratio improves.

The unit further comprises a confocal optics 42, typically operating ina telecentric mode, a relay optics 44, and an endoscopic probing member46. Elements 42, 44 and 46 are generally as known per se. It shouldhowever be noted that telecentric confocal optics avoidsdistance-introduced magnification changes and maintains the samemagnification of the image over a wide range of distances in the Zdirection (the Z direction being the direction of beam propagation). Therelay optics enables to maintain a certain numerical aperture of thebeam's propagation.

The endoscopic probing member typically comprises a rigid,light-transmitting medium, which may be a hollow object defining withinit a light transmission path or an object made of a light transmittingmaterial, e.g. a glass body or tube. At its end, the endoscopic probetypically comprises a mirror of the kind ensuring a total internalreflection and. which thus directs the incident light beams towards theteeth segment 26. The endoscope 46 thus emits a plurality of incidentlight beams 48 impinging on to the surface of the teeth section.

Incident light beams 48 form an array of light beams arranged in an X-Yplane, in the Cartasian frame 50, propagating along the Z axis. As thesurface on which the incident light beams hits is an uneven surface, theilluminated spots 52 are displaced from one another along the Z axis, atdifferent (X_(i), Y_(i)) locations. Thus, while a spot at one locationmay be in focus of the optical element 42, spots at other locations maybe out-of-focus. Therefore, the light intensity of the returned lightbeams (see below) of the focused spots will be at its peak, while thelight intensity at other spots will be off peak. Thus, for eachilluminated spot, a plurality of measurements of light intensity aremade at different positions along the Z-axis and for each of such(X_(i), Y_(i)) location, typically the derivative of the intensity overdistance (Z) will be made, the Z_(i) yielding maximum derivative, Z₀,will be the in-focus distance. As pointed out above, where, as a resultof use of the punctured mirror 40, the incident light forms a light diskon the surface when out of focus and a complete light spot only when infocus, the distance derivative will be larger when approaching in-focusposition thus increasing accuracy of the measurement.

The light scattered. from each of the light spots includes a beamtravelling initially in the Z axis along the opposite direction of theoptical path traveled by the incident light beams. Each returned lightbeam 54 corresponds to one of the incident light beams 36. Given theunsymmetrical properties of mirror 40, the returned light beams arereflected in the direction of the detection optics generally designated60. The detection optics comprises a polarizer 62 that has a plane ofpreferred polarization oriented normal to the plane polarization ofpolarizer 32. The returned polarized light beam 54 pass through animaging optic 64, typically a lens or a plurality of lenses, and thenthrough a matrix 66 comprising an array of pinholes. CCD camera has amatrix or sensing elements each representing a pixel of the image andeach one corresponding to one pinhole in the array 66.

The CCD camera is connected to the image-capturing module 80 ofprocessor unit 24. Thus, each light intensity measured in each of thesensing elements of the CCD camera, is then grabbed and analyzed, in amanner to be described below, by processor 24.

Unit 22 further comprises a control module 70 connected to a controllingoperation of both semi-conducting laser 28 and a motor 72. Motor 72 islinked to telecentric confocal optics 42 for changing the relativelocation of the focal plane of the optics 42 along the Z-axis. In asingle sequence of operation, control unit 70 induces motor 72 todisplace the optical element 42 to change the focal plane location andthen, after receipt of a feedback that the location has changed, controlmodule 70 will induce laser 28 to generate a light pulse. At the sametime it will synchronize image-capturing module 80 to grab datarepresentative of the light intensity from each of the sensing elements.Then in subsequent sequences the focal plane will change in the samemanner and the data capturing will continue over a wide focal range ofoptics 44, 44.

Image capturing module 80 is connected to a CPU 82 which then determinesthe relative intensity in each pixel over the entire range of focalplanes of optics 42, 44. As explained above, once a certain light spotis in focus, the measured intensity will be maximal. Thus, bydetermining the Z_(i) corresponding to the maximal light intensity or bydetermining the maximum displacement derivative of the light intensity,for each pixel, the relative position of each light spot along the Zaxis can be determined. Thus, data representative of thethree-dimensional pattern of a surface in the teeth segment, can beobtained. This three-dimensional representation may be displayed on adisplay 84 and manipulated for viewing, e.g. viewing from differentangles, zooming-in or out, by the user control module 86 (typically acomputer keyboard). In addition, the data representative of the surfacetopology may be transmitted through an appropriate data port, e.g. amodem 88, through any communication network, e.g. telephone line 90, toa recipient (not shown) e.g. to an off-site CAD/CAM apparatus (notshown).

By capturing, in this manner, an image from two or more angularlocations around the structure, e.g. in the case of a teeth segment fromthe buccal direction, from the lingal direction and optionally fromabove the teeth, an accurate three-dimensional representation of theteeth segment may be reconstructed. This may allow a virtualreconstruction of the three-dimensional structure in a computerizedenvironment or a physical reconstruction in a CAD/CAM apparatus.

As already pointed out above, a particular and preferred application isimaging of a segment of teeth having at least one missing tooth or aportion of a tooth, and the image can then be used for the design andsubsequent manufacture of a crown or any other prosthesis to be fittedinto this segment.

Reference is now being made to FIGS. 2A AND 2B illustrating a probingmember 90 in accordance with one, currently preferred, embodiment of theinvention. The probing member 90 is made of a light transmissivematerial, typically glass and is composed of an anterior segment 91 anda posterior segment 92, tightly glued together in an opticallytransmissive manner at 93. Slanted face 94 is covered by a totallyreflective mirror layer 95. Glass disk 96 defining a sensing surface 97is disposed at the bottom in a manner leaving an air gap 98. The disk isfixed in position by a holding structure which is not shown. Three lightrays are 99 are represented schematically. As can be seen, they bounceat the walls of the probing member at all angle in which the walls aretotally reflective and finally bounce on mirror 94 and reflected fromthere out through the sensing face 97. The light rays focus on focusingplane 100, the position of which can be changed by the focusing optics(not shown in this figure).

Reference is now being made to FIG. 3, which is a schematic illustrationof an endoscopic probe in accordance with an embodiment of theinvention. The endoscopic probe, generally designated 101, has a stem102 defining a light transmission path (e.g., containing a voidelongated space, being made of or having an interior made of a lighttransmitting material. Probe 102 has a trough-like probe end 104 withtwo lateral probe members 106 and 108 and a top probe member 110. Theoptical fibers have light emitting ends in members 106, 108 and 110whereby the light is emitted in a direction normal to the planes definedby these members towards the interior of the trough-like structure 104.The probe is placed over a teeth segment 120, which in the illustratedcase consists of two teeth 122 and 124, and a stamp 126 of a tooth forplacement of a crown thereon. Such a probe will allow the simultaneousimaging of the surface topology of the teeth segment from three anglesand subsequently the generation of a three-dimensional structure of thissegment.

Reference is now being made to FIG. 4. In this figure, a number ofcomponents of an apparatus generally designated 150 in accordance withanother embodiment are shown. Other components, not shown, may besimilar to those of the embodiment shown in FIG. 1. In this apparatus aparent light beam 152 is a combination of light emitted by a number oflaser light emitters 154A, 154B and 154C. Optic expander unit 156 thenexpands the single parent beam into an array of incident light beams158. Incident light beams pass through unidirectional mirror 160, thenthrough optic unit 162 towards object 164.

The different light components composing parent beam 152 may for examplebe different wavelengths, a different one transmitted from each of laseremitters 154A-C. Thus, parent light beam 152 and each of incident lightbeams 158 will be composed of three different light components. Theimage of the optics, or an optical arrangement associated with each oflight emitters may be arranged such that each light component focuses ona different plane, P_(A), P_(B) and P_(C), respectively. Thus in theposition shown in FIG. 3, incident light beam 158A bounces on thesurface at spot 170A which in the specific optical arrangement of optics162 is in the focal point for light component A (emitted by lightemitter 154A). Thus, the returned light beam 172A, passing throughdetection optics 174 yield maximum measured intensity of light componentA measured by two-dimensional array of spectrophotometers 176, e.g. a 3CHIP CCD camera. Similarly, different maximal intensity will be reachedfor spots 170B and 170C for light components B and C, respectively.

Thus, by using different light components each one focusedsimultaneously at a different plane, the time measurement can be reducedas different focal plane ranges can simultaneously be measured.

1. A method for determining surface topology of a portion of athree-dimensional dental structure, comprising providing an apparatuscomprising a focusing optics and a probing member having a front faceand a sensing face; the focusing optics defining one or more focalplanes forward of said sensing face in a position changeable by saidoptics; (a) providing an array of incident light beams propagating in anoptical path leading through said focusing optics and through saidprobing member, each light beam having its focus on one of said one ormore focal planes; the beams generating a plurality of illuminated spotson the structure, said apparatus being hand held to direct said opticalpath at a desired location with respect to said dental structure viasaid sensing face; (b) detecting intensity of returned light beamspropagating from each of the plurality of illuminated spots along anoptical path opposite to that of the incident light; (c) repeating steps(a) and (b) a plurality of times, each time changing position of thefocal plane relative to the structure; (d) for each of the illuminatedspots, determining a spot-specific position, being the position of therespective focal plane yielding a maximum measured intensity of arespective returned light beam; and (e) generating data representativeof the topology of said portion.
 2. The method according to claim 1,wherein said method is applied to determining surface topology of teethin an oral cavity of a patient.
 3. The method according to claim 1,wherein said array of incident light beams are produced by splitting aparent light beam.
 4. The method according to claim 1, comprising instep (a) internally folding said incident light beams in said probingmember prior to directing said incident light beams towards the dentalstructure via said sensing face.
 5. The method according to claim 1,comprising in step (a) internally folding said incident light beams aplurality of times in said probing member prior to directing saidincident light beams towards the dental structure via said sensing face.6. The method according to claim 1, comprising in step (a) deviatingsaid incident light beams passing through said probing member from saidfront face to said sensing face by a net deviation substantially greaterthan 0 degrees.
 7. The method according to claim 1, comprising in step(a) deviating said incident light beams passing through said probingmember from said front face to said sensing face by a net deviationsubstantially greater than 90 degrees.
 8. The method according to claim1, wherein said optical paths of said incident light beams through saidprobing member pass through said front face substantially perpendicularthereto.
 9. The method according to claim 1, wherein said optical pathsof said respective incident light beams are substantially parallel withrespect to one another within said probing member.
 10. The methodaccording to claim 1, wherein said focusing optics comprises confocaloptics and wherein in step (c) the position of the focal plane ischanged each time by displacing said confocal optics.
 11. The methodaccording to claim 1, wherein in step (a) an illumination unit providessaid incident light beams and in step (b) a detector detects saidintensities of returned light beams, and wherein the optical path ofsaid incident light beams from said illumination unit to said sensingface excludes passage through said detector.
 12. The method according toclaim 11, wherein said apparatus comprises an optical component, saidoptical component separating the incident light beam optical path fromthe returned light optical path thereat, allowing transmission of saidincident light beams therethrough towards said focusing optics in step(a), while redirecting said returning light beams to said detector instep (b).
 13. The method according to claim 12, wherein said separatingof said incident light beam optical path from the returned light opticalpath occurs between the illumination unit and the focusing optics. 14.The method according to claim 1, wherein each said returned light beamcorresponds to a said incident light beam.
 15. The method according toclaim 1, wherein in step (c), the position of the focal plane relativeto the structure is changed in a telecentric manner.